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        .callout {
            position: relative;
            height: 40px;
            width: 40px;
            border: 1px solid #ccc;
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        .callout::before {
            content: "";
            position: absolute;
            top: -.4em;
            left: 1em;
            padding: .35em;
            background: inherit;
            border: inherit;
            border-right: 0;
            border-bottom: 0;
            transform: rotate(45deg);
        }
        /* 这个inherit 关键字对于背景色同样非常有用。举个例子，在创建提
        示框的时候，你可能希望它的小箭头能够自动继承背景和边框的样式 */
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